(Source: http://www.intelligentenergysolutions.com/Sustainable-Energy-Blog.html)

Why do renewable energy companies recommend underfloor heating for heat pump installations?

The answer is simple. Underfloor heating runs at lower temperatures than conventional radiators. Heat pumps prefer to output lower temperatures as this makes them more efficient.

Is underfloor heating a necessity?

This depends on the heat pump used, some high quality heat pumps (used by Intelligent Energy Solutions) can output temperatures up to around 55 degrees. This is a scolding temperature and most radiators will give the required output at this temperature. The reason that the radiator will function at a lower temperature is because most radiators are oversized by the installing plumber anyway.
The efficiency of the heat pump can lead to massive savings when compared to both gas and oil fossil fuel boilers.

However, in an ideal world the heat pump will run most effectively with underfloor heating.